The proposed studies aim to identify and analyze the sensorimotor transformations by which the central nervous system achieves accurate control of multijoint movements directed at visual targets. They address the specific question of how information about the location and state of the limb and about the target operate to assure the accurate programming and control of movement. Experiments are to be done with normal subjects, with patients affected by severe large-fiber neuropathy and with patients with circumscribed cerebellar lesions. A novel procedure, the timed response paradigm, will be used to analyze the time course and nature of the processes by which the direction and extent of movement are specified. It is hypothesized that a fundamental cause of the inaccuracy of both sensory neuropathy and cerebellar dysmetria is a failure to take state-dependent and dynamic interactions into account in both the programming and control of multijoint movement. This hypothesis is based on preliminary results which indicate that distinctive errors in direction and extent are attributable to simplifications in the kinematic and dynamic transformations by which target location is eventually converted into the muscle commands needed to bring the hand to the target. These systematic programming errors are greatly magnified in patterns deprived of proprioceptive sensation but can be partially corrected by vision of the limb prior to movement. This effect is, however, transitory and its time course will be determined. The proposed studies will also determine the mechanisms of the impairment in interjoint coordination shown in preliminary work to follow the loss of proprioceptive sensation. In its latter phase, the proposed work will characterize the movement errors made by patients with medial and lateral lesions of the cerebellar hemispheres. The role of these structures in short-term learning may allow them to participate in the generation and continual refinement of internal models of the limb.